Should we add a new capability of Executable?

[kayoch1n]

Feature Description

Should we add a new capability to detect whether the filesystem supports the Executable bit? If there is a need, I am willing to open a PR and work on this.
This is related to go-git #1712. I am going to implement this by flipping the exec bit.

Once the Executable capability is implemented, go-git will be able to detect and set the correct value for core.filemode during init/clone.

Related: go-git #771

[pjbgf]

I have just assigned it to yourself, please go ahead and propose a PR.